Electric valve protective system



1945 H. WENOGRAD ELECTRIC VALVE PROTECTIVE SYSTEM 4 Sheets-Sheet 1 Filed Feb; 27, 1942 Dec. 25, 1945.

H. WINOGRAD ELECTRIC VALVE PROTECTIVE SYSTEM Filed Feb. 27, 1942 4' Sheets Sheet 2 Dec. 25, 1945. wmo 2,391,812

ELECTRIC VALVE PROTECTIVE SYSTEM Filed Feb. 27, 1942 4 Sheets-Sheet 3 Dec. 25, 1945. wlNOGRAD 2,391,812

ELECTRIC VALVE PROTECTIVE SYSTEM Filed Feb. 27, 1942 4 Sheets-Sheet 4 mam 1v Patented Dec. 25, 19 45 assnsia/ ELECTRIC VALVE rno'racrrva SYSTEM Harold Winograd, Milwaukee, Wis., assignor to Allis-Chalmers Manufacturing Company, Milwaukee, Wis., a corporation of Delaware Application February 21, 1942, Serial No. 432,577

20 Claims.

This invention relates in general to improvements in protective systems for arcing devices,

and more particularly to means for preventing operation of an electric valve or other electron discharge device of the vapor arcing type under circumstances conducive to failure of the valve action of the device.

In the operation of electric valves of the vapor arcing type each valve is generally required to carry current by way of arcs established between the electrodes of the valve during predetermined periods. At the end of each period the arc is generally interrupted by sudden reversal of the voltage impressed on the electrodes. At the moment of interruption of the arc the operating vapor contained within the valve is in ionized condition. The density of ionization in the arc path, i. e., the number of vapor ions per unit volume, is generally highest near the cathode and lowest in the vicinity of the arcing surface of the anode. Upon impression of a reverse voltage on the electrodes of the valve, the vapor ions are attracted toward'the anode. An ionic current is thus established in the vapor of the valve by the ions traveling to the anode, where they lose their charges to cause a corresponding current to flow through the circuits connected with the valve. i

The value of the ionic current is maximum immediately after reversal of the voltage appearing between the electrodes. It rapidly decreases thereafter to a negligible value because the anode, which then functions as negative electrode, normally cannot emit the electrons required for sustaining a continuous discharge.

The initial value of the ionic current depend on.

the value of the anode to cathode voltage immediately after its reversal, on the configuration of the valve, and on the density of ionization in the vicinity of the anode arcing surface. The

density of ionization depends in turn on the temperature of the different elements of the valve and on the magnitude of the load current previously carried by the arc.

If the ionic current density exceeds a predetermined value, depending on the design of the valve, the voltage gradient at the surface of the anode may reach a sufiicient value to cause emission of electrons thereat.

Under such conditions, the valve action of the valve eventuall fails and the ionic current is followed by a reverse are or backfire. The backfire current disturbs the operation of the valve and must be extinguished before normal operation of the valve can be resumed, as is Well may be controlled in dependence upon the value v known. If the ionic current approaches a value at which failure of the valve action is likely to result, the operation of the valve should be interrupted or at least afiected in such manner as to cause the ionic current to be reduced. This result may be obtained, for example, by reducing the magnitude of the load current through the valve or by lowering the temperature of the valve.

Likewise, the operation of an electric switch of the ionic current flowing between the switch contacts upon establishment and extinction of an arc therebetween to prevent reignition of the arc.

It is therefore an object of the present invention to provide a protective system for Vapor arcing devices such as an electric switch or an electric valve of the vapor type, which is operable in response to the density of ionization of the vapor within the device.

Another object of 'the present invention is to provide a protective system for a vapor arcing device preventing rise of the ionic current of the device to a value which may cause reignition of the arc.

Another object of the present invention is to provide a system for controlling the flow of current through a vapor arcing device in dependence upon the magnitude of the ionic current of the device.

Another object of the present invention is to provide a system for measuring the value of the ionic current flowing through a device of the vapor arcing type.

Objects and advantages other than those above set forth will be apparent from a consideration of the following description when taken in connection with the accompanying drawings, in which: i

Fig. 1 diagrammatically illustrates one embodiment of the present invention for opening the circuit of a rectifying system of the electric valve type upon the occurrence of excessive ionic current in one of the valves or of an excessive voltage drop in another one of the valves;

Fig. 2 diagrammatically illustrates another embodiment of the present invention for rendervalue of the ionic current or one of the valves of the system; and

Fig. 4 diagrammatically illustrates another embodiment of the present invention for regulating the supply of current to an inverting system of the electric valve type in dependence upon the value of the ionic current of one of the valves.

It will be understood that elements of any of the embodiments herein illustrated ma also be combined with elements of the other embodiments to form further embodiments of the present invention.

Referring more particularly to Fig. 1 of the drawings by characters of reference, an alternating current supply circuit 6 is to be connected with a direct current load circuit I through a converter 8 of the electric valve type comprising a transformer 9 and a plurality of electric valves II to I6. Transformer 3 comprises a primary winding II connectable with the conductors of circuit 6 through a switch I8 serving to interrupt the flow of current through the system. Winding I1 is inductively related with a secondary winding I9 preferably divided into a plurality of phase portions. Each of the phase portions of winding I3 is assumed to be connected across circuit I through two valves in series, such as valves I3, I6, as may be found advantageous when the voltage circuit I is of relatively high value.

Valves II to I6 are assumed to be of the single anode vapor type in which ionizable operating vapor is supplied by evaporation from a mercury cathode. The anode 2I of valve I3 is connected with one of the terminals of winding I9 and the cathode 22 of valve I6 is connected with the positive conductor of circuit I. The cathode'23 of valve I3 is connected with the anode 24 of valve I 6. The valves are provided with the usual means (not shown) for rendering the cathodes intermittently emissive or for maintaining the oathodes continuously emissive as is well known.

For measuring the density of ionization of the vapor in the vicinity of the arcing surface of anode 2| during the idling periods of valve I3, valve I6 is connected in parallel with an impedance element such as a resistor 25. Resistor 25 receives a part of the ionic current of valve I3 through its connection with cathode 23 of the valve. The resistance of resistor 25 is preferably made relatively low so as to divert the greater portion of the ionic current of valve I3 from valve I6. A capacitor 26 is connected across an adjustable portion of resistor 25 through a rectifler 21 -of any suitable type and an adjustable resistor 28 of relatively high total resistance. A discharge resistor 29 is connected in parallel with capacitor 26 through an ammeter 30. A battery II and a voltage divider 32 connect the capacitor between the control electrode 33 and the cathode 34 of an electric valve 35 of the discontinuously controllable vapor type. The anode 36 and the cathode 34 of valve 35 are connected through a battery 31, the coil of a relay 38, and a switch 39. The contacts of relay 3! control the connection of the trip coil of switch It with a battery.

For measuring the arc drop in valve I6 during the flow of load current through the valve, a portion of resistor 25 is connected between the control electrode and the cathode of a valve 42 of the continuously controllable high vacuum type. This connection preferably comprises a battery 43 bridged by a voltage divider 44. The anode and cathode of valve 42 are connected through a battery 45, the coil of a relay 46 and a measuring device such as an ammeter 41. The contacts of relay 46 are connected in'parallel with the contacts of relay 36 to cause opening of switch l8 upon operation of either of the two relays.

In operation, switch I8 being closed and circuit 6 being energized from a suitable generator (not shown), winding I9 impresses on the different pairs of serially connected valves voltages which bring the anodes of the diiferent pairs sequentially at positive potentials with respect to the potentials of the associated cathodes. As a result thereof, the different pairs of valves sequentially transmit load current from winding I9 to circuit I by way of intermittent arcs established between the electrodes of the valves. When load current is flowing through valves I3, I6, for example, the entire current flows momentarily through valve I3. Substantially the entire current also flows through valve I6, the remainder flowing through resistor 25.

The value of the current flowing through resistor 25 is proportional to the arc drop in valve I6, which should not exceed a predetermined value to enable the valve to function without disturbance. The connection of the control electrode of valve 42 with resistor 25 causes the conductivity of valve 42 to vary with the value of the arm drop in valve I6. Battery 45 therefore supplies to relay 46 and meter 41 a current varying with the value of the arc drop in valve I6, which may be read on meter 41. Between current conductive periods of valve I6, the control electrode of valve 42 is at a negative potential and renders the valve substantially non-conductive. The current of valve 42 is therefore pulsating, but the inertia of meter 41 causes the meter to respond to the average value of the current of valve 42 if the meter is of the permanent magnet type. Relay 46 is responsive to the root mean square value of the current of valve 42.

During the arcing periods of valves I3, I6 considered above, no voltage is impressed from resistor 25 on capacitor 26 because of the interposition of rectifier 21 in the connection therebetween. The arcing periods of valves I3, I6 end when the voltage impressed from winding I! on circuit 1 through valves I3, I6 decreases below the value of the voltage impressed on circuit I through another pair of valves, thereby interrupting the flow of load current through valves I3, I6 while the load current is transferred to the succeeding pair of valves I2, I5. The voltage impressed on the electrodes of valves I3, I6 is thus reversed.

Upon interruption of the load current through valve I3 an ionic current appears therein by migration of the vapor ions to anode 2I under the 4 action of the reverse voltage impressed from winding IS on the electrodes of the valve. A circuit for the ionic current is completed from anode 2I through winding IS, the then operating valves I2 and I5 and through valve I6 and resistor 25 in parallel to cathode 23. The part of the current flowing through valve I6 is in the form of an ionic current similar to that of valve I3. A portion of the current flowing through resistor 25 is diverted through rectifier 21 and resistor 28 to charge capacitor 26.

The flow of ionic current consists of impulses occurring after the termination of each arcing period of valves I3, I5. In the interval between these impulses, rectifier 2'I prevents discharge of capacitor 26 through resistor 25. The capacitor. however, discharges slowly through resistor 23 and ammeter 30.. During steady state operation of the valves at constant load, the loss of charge or capacitor 26 through resistor 29 during each cyle of the voltage of circuit 8 is compensated by a charge of equal value during each period of ionic current flow to maintain the voltage of capacitor 26 substantially constant at a value corresponding to the peak value of the ionic current. Am-

meter 30, which measures the value of the volta e of capacitor 26, then also measures the peak value of the ionic current.

When the operating conditions of valve l3 vary, the peak value of the ionic current varies also. Resistor 28 is, however, preferably adjusted at such high value that the charge of capacitor 26 does not vary to the same extent as the ionic current from one cycle to the next. The capacitor voltage therefore assumes a value which is representative of the average of the peak values of the successive ionic current impulses flowing between cathode 23 and anode 2| during a number of cycles, depending upon the adjustment of resistor 28. An accidental momentary increase in the value of the ionic current does not materially change the capacitor voltage.

If the density of ionization in valve l3 increases beyond the safe limit as a result of an excessive increase in the load current'of the valve, for example, the ionic current impulses continually reach an excessive peak value. If these conditions prevail for an appreciable length of time, the charge of capacitor 26 increases at a rate depending on the adjustment of resistor 28, to gradually reach a value corresponding to the increased peak value of the ionic current. When the voltage of capacitor 26 reaches a predetermined value, control electrode 33 reaches its critical. potential with respect to the potential of cathode 34 to render valve 35 conductive. Current then flows from battery 31 through valve 35, switch 39 and the coil of relay 38 to cause actuation of the relay. It will be understood that ionic current impulses of excessive magnitude may also take lace during the idling periods of valve i3 at other moments than immediately after termination of its arcing periods. Such impulses may appeanior example, if the maximum reverse voltage 1m pressed from/Winding l9 on valves l3, i is higher than the voltage the valves are designed to withstand.

The flow of load current of excessive magnitude through the valves will also cause the arc drop in valve IE to increase to a value which may be excessive. Voltage divider 44 and the lower tap of resistor 25 are so adjusted that when the arc drop in valve i5 becomes excessive, the flow of current from battery 45 through valve 42, meter 41 and the coil of relay 46 becomes suiilclent to cause actuation of the relay.

Depending upon the relative rates. of increase of the ionic current of valve l3 and of the arc drop in valve l6, relay 38 or'relay 46 will operate first to cause opening of switch H3. The system is thereby prevented from operatingunder conditions likely to cause disturbances ll! the operation of the valves or'damage to their elements. If relay 38 operates, it may be reset only by opening switch 39 because control electrode 33 cannot interrupt the flow of current through valve 35.

In the embodiment illustrated in Fig. 2, converter 8 is assumed to comprise only valves H to |3 for connecting the terminalsgof winding IS with the positive conductor of circuit 1. The valves are assumed to be provided with control electrodes for controlling their conductivity.

Valve l3, for example, is

electrode or grid 49 operable for rendering the valve non-conductive upon connection of the control'electrode with cathode 23 through a resistor 5|, a battery 52, and the contacts of relay 38. In the present embodiment each'of the valves is 1 provided with a probe electrode, such as electrode 53 arranged between the arcing surface of anode 2| and control electrode 49.

Electrode 53 may be a smallsolid or perforated pl-ate disposed in front of the anode arcing surface. It is insulated from anode 2| and is provided with any suitable connections extendin through the casing of valve l3 or through the stem of the anode. The location'of electrode 53 is so chosen that the density of ionization of the vapor adjacent the electrode surface is approximately the same as that adjacent the anode arcing surface. To obtain this result, electrode 53 should be well shielded from the cathode. Control electrode 49 may provide suificient shielding for electrode 53, but additional shields may be provided if necessary. The surface of electrode 53 facing the cathode may also be covered with heat resisting insulating material to separate the electrode from the highly ionized portion of the vapor within valve I 3. Resistor 25 preferably connects cathode 23 with the probe electrodes of vented by rectifier 21. When the voltage between anode 2| and cathode 23 reverses uponinterruption of the arc therebetween, the flow of ionic current from cathode 23 to anode 2| takes place through resistor 25, rectifier 21, resistor 54, electrode 53 and the ionized vapor in the space adjacent anode 2| and electrode 53. During each cycle of the voltage of circuit 6, ionic currents flow through resistor 25 to the probe electrodes of valves II to 3 following the sequential establishment and interruption of intermittent arcs in the different valves. 5

If capacitor 26 is omitted, as shown in Fig. 2, the potential of control electrode 33 varies continually with respect to that of cathode 34 in response to the flow of ionic current through resistor' 25, instead of being maintained substantially constant by the charge of the capacitor.

As long as the potential of control electrode 33 remains below its critical value, valve 35 remains non-conductive. If the largest of the ionic currentsof valves II to |3 reaches a predetermined peak value depending upon the adjustment of the 'tap of resistor 25 and of voltage divider 32, conconnected in series with resistor 25 and valve 35 may be omitted. The relay then operates in dependence upon the root-mean square value of the ionic current rather than in dependence upon its peak value.

In the embodiment illustrated in Fig-3, the

anodes of valves I to l3 are assumed to be enprovided with a control closed within a common casing, cathode 23serving as common cathod for the three valves. Except -upon occurrence of unbalances in the currents of the different anodes, the density of ionization then has substantially the same value in the vicinity of all the anodes and one probe electrode is sutilcient to detect abnormal operating conditions of all the anodes. Electrode 53 may again be disposed in front of the arcing surface of anode 2| or may be inserted instead in a recess provided therefor in the anode arcing surface.

In the present embodiment resistor is connected between electrode 53 and anode 2|. Valve is replaced by a valve 55 of the continuously controllable high vacuum type having an anode 55, a cathode 51 and a control electrode 58.- Battery 31 is connected with anode 56 and cathode 51 through meter 30 and through an actuati coil 55 of a voltage regulator 5 l The regulator serves to control the magnitude of the negative bias potential component impressed on the control electrodes of valves II to l3 from a. battery 52. The main actuating coil 63 of the regulator may be connected between the conductors of circuit 1 through a resistor 54. Coils 55, 53 jointly actuate a rocking sector 55 cooperating with a resistor 55 connected with battery 52 and with cathode 23 through a resistor 61. Sector 65 is connected with the neutral point of the secondary winding 55 of a control transformer 69 having a primary winding ll energized from circuit 6. The terminals of winding 58 are connected with the control electrodes of valves II .to I! through resistors such as resistor 12.

The operation of valves II to IS in the present embodiment diilers from that above described in that each valve carries current only after its control electrode has reached a predetermined critical potential. The control electrodes of the valves are maintained at a potential below the critical value by battery 62 and are sequentially caused to pass through .the critical potential by transformer 68. Coil 63 of regulator 5i causes the rocking sector to vary gradually the potential component impressed on the control electrodes from battery 52 to maintain the voltage of circuit 1 substantially constant. Voltage divider 32 is preferably so adjusted that control electrode 58 maintains valve 55 substantially non-conductive at least when converter 8 is not operating.

During the operating periods of anode 2! some current may flow from anode 21 through resistor 25 and electrode 53 in parallel with a portion of the arc. The potential impressed on control electrode 55 from resistor 25 as the result of the flow of such current tends to maintain valve 55 non-conductive. 'At the end of each arcing period of anode 2|, the voltage impressed between anode 2l and cathode 23 assumes immediately a high reverse value because of the delay imposed on the operating periods of valves H to I; by their control electrodes. The ionic current therefore tends to be larger than when the operating periods of the valves are not controlled.

Ionic current flows from cathode 23 through the ionized vapor of valv ii to electrode 53, and at least a part of this ionic current flows therefrom through resistor 25 to anode 2|. The voltage between control electrode 58 and cathode 51 therefore follows the variations of the ionic current. Voltage divider 32 may be so adjusted that the conductivity of valve 55 also follows the variations-of the ionic current to cause the current in coil 55 to vary in the same manner. The

voltage divider may also be so adjusted as to cause valve 55 to become materially conductive only when the ionic current exceeds a predetermined value, so that coil 52 remains substantially ineil'ective when the ionic current is not excessrve.

when coil 55 receives current, it acts jointly with coil 3 on rocking sector 85 to depress the potential of the control electrodes of valves II ,to IS. The conductivity of the valves is thereby gradually decreased to decrease the outputvoltage of converter I and thereby reduce the load current of the system when the ionic current oi. the anode becomes excessive. The value of the ionic current may be read on ammeter II; which is preferably so calibrated as to take into consideration the characteristics of valve 55 and the adjustment of voltage divider 22. The action of coil 55 takes place as if the current of coil 55 were added to the relatively much larger current of coil 52. Because of the inertia of the regulator elements, regulator I does not respond to the momentary variations in the value of the ionic current and respons only to substantially its average value. Meter 25 indicates substantially the average value of the imic current'if the meter is of the premanent magnet type.

In the embodiment illustrated in Fig. 4, valves H to II are assumed to be operable to invert direct current from circuit i into alternating current to be supplied to circuit 5. Asia well known, in an inverter the periods during which the anode to cathode voltage is of reverse polarity are very short. The periods of positive energizatlon of the control electrodes are therefore preferably made shorter than the operating periods of the anodes to prevent reignition of the anodes at the end of the periods of reverse voltage. This result may be obtained, for example, by connecting winding H with circuit i through suitable impedance means Ill and by so dimensioning the core 13 of control transformer 59 as to cause the core to be saturated. Winding 65 then impresses on the control electrodes of valves II to l3 positive voltage impulses of peaked wave form of which the base is suppressed by battery 62. In addition, the space adjacent each anode must become deionized completely during the short period of reverse voltage, which requires that the density of ionization, and consequently the peak value of the anode ionic current, be maintained low. 4

In the present embodiment the ionic current is measured by connecting resistor 25 in series with rectifier 21 between cathode 25 and control electrode is. The control electrode also serves as probe electrode and therefore should be well shielded from cathode 23. Resistor I2 may be chosen of high resistance relative to resistor 25 to reduce the value of the periodic unidirectional current circulating from winding 58 through battery 62, resistor 25, rectifier 21 and resistor I2. Resistor 12 may instead be connected in series with a rectifier II, which is bridged by a resistor 15 of high value to enable battery 62 to impress a negative potential on control electrode 4!. Circuit I is assumed to receive rectified current from an alternating current circuit 15 through a converter 'Il similar to converter 5. The output of converter ll may be controlled by means of a rocking sector regulator II similar to regulator 5| of the embodiment illustrated in Fig. 3. Resulator 15, however, is provided with a single actuating coil I5 energized from battery 31 through valve 55 and ammeter 35.

In the present embodiment the conductivity of valves I I to i3 is assumed to have a constant adjustment. The flow of ionic current from cathode 23 to anode 2! takes place through resistor 25, rectifier 21, control electrode 49 and the ionized vapor contained within valve l3 between electrode 49 and anodell. The value of the ionic current is measured by ammeter 30 in the same manner as in the embodiment illustrated in Fig. 3. The current of meter 30, which also flows through coil 19, causes regulator 18 to regulate the output of converter 11 in a manner similar to that above described with respect to converter 8 in the embodiment illustrated in Fig. 3.

Voltage divider 32 may be so adjusted that coil 19 receives a current following all the variations of the ionic current to cause the current supplied to converter 8 to be gradually decreased when the ionic current of valve l3 increases. Regulator 18 then operates in response to substantially the root-mean square value of the ionic current. Voltage divider 32 may also be so adjusted that valve 55 becomes conductive only when the ionic current exceeds a predetermined value to cause regulator 18 to operate in response to the peak value'of the ionic current.

Although but a few embodiments of the present invention have been illustrated and described, it

will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention or from the scope of the appended claims.

It is claimed and desired to secure by Letters Patent:

1. The combination of an electric arcing device comprising electrodes and an ionizable vapor for the transmission of electric current by way of arcs between said electrodes, means for controlling the operation of said device, and means variably operable in dependence upon variations in the density of ionization of said vapor at a predetermined point Of said device only immediately after interruption of an arc between said electrodes for controlling the operation of the first said means.

2. The combination of an electric valve comprising anode and cathode electrodes and an ionizable vapor for the transmission of electric current by way of arcs between said electrodes, means for controlling the operation of said valve, and means variably operable in dependence upon variations in the density of ionization of said vapor adjacent the arcing surface of'said anodeonly immediately after interruption of an are between said electrodes for controlling the operelectrodes, and means for controlling the opera- .tion of said valve comprising means operable in dependence upon the average value of the ionic current flowing between said electrodes only upon interruption or saidarcs by reversal of the volttage impressed on said electrodes.

5. The combination of an electric valve comprising electrodes and an ionizable vapo for the transmission of electric current by way of arcs between said electrodes, 9. ,source of alternating current connected with said electrodes'to cause establishment of intermittent arc-s between said electrodes, and means ior controlling the operation of said valve comprising means operable in dependence upon the peak value or the ionic current nowing between said electrodes only upon interruption or said arcs by reversal of the voltage impressed on said electrodes.

6. The combination or an electric valve comprising electrodes and an ionizable vapor for the transmission of electric current by way 01' arcs between said electrodes, a source of alterhatmg current connected with said electrodes to cause establishment of intermitent arcs between said electrodes, and means 101 controlling the operat on or said valve comprising means op rable in dependence upon the average or the pea: values or the successive ionic current impulses nowing between said electrodes only upon interruption of the successive said arcs by reversal or the voltage impressed on said electrodes.

'1. The combination or a plurality of electric valves each comprising electrodes and an ionizable vapor for the transmission OI electric current by way 01' arcs between said electrodes, 2 source or polypnase alternatmg current connected with Said electrodes to cause sequential establishment or intermittent arcs tween the electrodes oi the diiferent said valves j and means for. controlling the operation OI said valves comprising means selectively responsive to the magnitude oi the lar est or the coherent iomc curthe ionic current nowmg between said electrodes current connected with said electrodes to cause I establishment of intermittent arcs between said only upon interruption or said arcs by reversal oi; the voltage impressed on said electrodes.

9. The combination or an electricvalve comprising electrodes and an ionizable vapor for the transmission of electric current by way of arcs between said electrodes. a source of alternating current connected with said electrodes to cause establishment oi intermittent arcs between said electrodes, and means rendering said valve nonconuuctive comprising a control element of said valve and means for controlling the potential of said element responsive to the magmtude of the ionic current flowing between said electrodes upon interruption of said arcs by reversal of the voltage impressed on said electrodes.

The combination of an electric valve comprising electrodes and an ionizable vapor for the transmission of electric current by way of arcs between said electrodes, at source of alternating current connected with said electrodes to cause establishment of intermittent arcs between said electrodes, and means for gradually regulating the conductivity of said valve comprising a control element of said valve and means for controlling the potential of said element responsive to the magnitude of the ionic current flowing between said electrodes only upon interruption oi said arcs by reversal of the voltage impressed on said electrodes.

11. The combination of an electric valve comprising main electrodes and an ionizable vapor for the transmission of electric current by way oi. arcs between said main electrodes, a source of alternating current connected with said main electrodes to cause establishment of intermittent arcs between said main electrodes, and means for controlling the operation of said valv comprising an auxiliary electrode of said valve, a connection between said auxiliary electrode and one of said main electrodes to provide a path for the flow of a part of the ionic current flowing between said main electrodes upon interruption of said arcs, and means responsive to the magnitude of only the part of said ionic current flowing through said connection.

12. The combination of an electric valve comprisingelectrodes and an ionizable vapor for the transmission of electric current by way, of arcs between said electrodes, a source of electric current connected with said electrodes to cause establishment of intermittent arcs between said electrodes, a second electric valve serially connected with the first said valve to conduct substantially the entire current flowing through said arcs, and means for controlling th operation of said valves comprising an impedance element connected in parallel with the second said valve to carry a part of the ionic current flowing between said electrodes upon interruption of said arcs by reversal of the voltage impressed on said electrodes, and means responsive to the magnitude of the part of said ionic current flowing through said impedance element.

13. The combination of an electric valve comprising electrodes and an ionizable vapor for the transmission of electric current by way of arcs between said electrodes, 2. source of electric current connected with said electrodes to cause establishment of intermittent arcs between said electrodes, a second electric valve serially connected with the first said valve to conduct substantially the entire current flowing through said arcs, and means for controlling the operation of said valves comprising an impedance element connected in parallel with the second said valve to carry a part of the ionic current flowing between said electrodes upon interruption of said arcs by reversal of the voltage impressed on said electrodes and to carry a current proportional to the arc drop in the second said valve while arcs are established in said valves, and means selectively responsive to the magnitudes of the said currents flowing through said impedance element.

14. Means iormeasuring the ionic current flowing through an electric arcing device upon interruption of an arc therein by reversal of the voltage impressed on the electrodes of the device comprising an electric valve serially connected with said device, an impedance element connected in parallel with said valve, and means responsive to the magnitude of the flow of current through said impedance element.

15. Means for measuring the peak valu or the ionic current impulses flowing through an electric arcing device upon interruption of an arc in the device by reversal of the voltage impressed on the electrodes 01' the device comprising a resistor connected with one of the electrodes of the device to carry a part of the ionic current of the device, a capacitor, rectifying means connecting said capacitor with said resistor, and means for measuring the voltage of said capacitor.

16. Means for measuring the average peak valu of the ionic current impulses flowin through an electric arcing device upon interruption oi the arc in the device by reversal of the voltage impressed on the electrodes of the device comprising a resistor of relatively low resistance connected with one of the electrodes of the device to carry a part of the ionic current of the device, a capacitor, rectifying means connecting said capacitor with said resistor, means for measuring the voltage of said capacitor, and a resistor of relatively high resistance in the connections between the first said resistor and said capacitor for reducing the variations of th voltage of said capacitor in response to variations in the value of said ionic current.

17. Means for measuring the ionic current flowing through an electric valve of the vapor arcing type upon interruption of the arc in the valve by reversal of the voltage impressed on the anode and cathode of the valve comprising an auxiliary electrode arranged adjacent the arcing surface of the anode, a resistor and a rectifying device serially connected between said auxiliary electrode and said cathode, and means responsive to the magnitude or the current flowing through said resistor.

18. Means for measuring the ionic current flowing tlirough an electric valve of the vapor arcing type upon interruption of the arc in the valve by reversal of the voltage impressed on the anode and cathode of the valve comprising an auxiliary electrode arranged adjacent the arcing surface of the anode, a resistor connected between said auxiliary electrode and said anode, and means responslve to the magnitude of the current flowing through said resistor.

19. The combination of an electric valve comprising main electrodes and an ionizable vapor for the transmission of electric current by way of arcs between said main electrodes, a source of alternating current connected with said main lectrodes to cause establishment of intermittent arcs between said main electrodes, and means for controlling the operation of said valve comprising a control electrode of said valve, means for impressing potential on said control electrode to control the moments of initiation of said arcs, a resistor and a rectifying device serially connected between said control electrode and said cathode to provide a path for the flow of a part of the ionic current flowing between said main electrodes upon interruption of said arcs by re- 

